class for "parent-children relation".
initialize(parent)
parent
is a parent for self.
parent
my parent.
add(c)
add a child for self.
delete(c)
delete one of my childen.
die
self and children die.
killchildren
children die.
close
die and set "closed" true.
closed?
canclose?
can I close?
showtree(indent=0)
print tree. for debug.